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Opal is composed of hydrated silica. It is an amorphous silicate, meaning it does not form crystals. Opals are structurally composed of tiny spheres of silica which, when arranged in perfect geometry, create the famous play of color associated with the stones. The symmetrical arrangement of silica balls acts like a prism, which allows the rainbow spectrum of colors to be visible to the naked eye.
Fire opal is a transparent to translucent opal, with warm body colors of yellow to orange to red. Although it does not usually show any play of color, occasionally a stone will exhibit bright green flashes. The most famous source of fire opals in Mexico is in the state of Queretaro; these opals are commonly called Mexican fire opals. Fire opals that do not show a play of color are sometimes referred to as jelly opals.
Cantera Opal is a gemstone formed from Rhyolite, which is a kind of igneous rock. When magma cooled on the earth surface, traces of gas and steam escaped from the stone, leaving cavities. Eventually, opal would form within these volcanic rock cavities, creating what we know as Cantera Opal.
